pgib8
Member level 2
Hi, I'm looking at PCB for switching AC mains voltage, and I'm only looking at the contacts side and not worry about the solenoid side, which includes a free-wheeling diode for the solenoid. I believe it is a poor design because there is no protection across the contacts and there is also a power supply on the same board that is producing voltage spikes when the relay switches.
I only notice this when I switch an inductive load, for which I'm using a 75W blower fan.
My theory was that when the contacts open, the inductive load produces a high voltage that arcs across the contacts. I'm pretty sure I have confirmed this by looking at the voltage across the contacts on the scope. See below:
First what it looks like when switching off an incandescent light bulb (yellow trace is the contact voltage):
Now the same thing with the fan instead:
So when I zoom in on that spike I see this:
And when I zoom in further, at the beginning of it:
So here is my question, if this is really because of arcing, I would like to understand this waveform better.
I imagine at the beginning the voltage increases because the contacts have already separated at this point and the inductive load is building up the voltage as the parasitic capacitance gets charged up by it. Then when the trace comes vertically straight up is where the arc occurs. The ringing that follows is either an artifact from the measurement or some kind of natural oscillation in the circuit like a spark-gap transmitter but I'm not too worried about this. I assume the arc extinguishes right when the voltage starts to build up again.
So the thing I don't understand what are the flat spots. Later on in the waveform they become really pronounced, take a look:
It appears to me that during the flat spots, the contacts have about -160V across and are just sitting there and then out of the blue it decides to arc again? Why wouldn't the voltage keep increasing and then once it reaches a maximum then right away the vertical line happens? Or maybe I'm completely misunderstanding what's going on and any help would be greatly appreciated!
I only notice this when I switch an inductive load, for which I'm using a 75W blower fan.
My theory was that when the contacts open, the inductive load produces a high voltage that arcs across the contacts. I'm pretty sure I have confirmed this by looking at the voltage across the contacts on the scope. See below:
First what it looks like when switching off an incandescent light bulb (yellow trace is the contact voltage):
Now the same thing with the fan instead:
So when I zoom in on that spike I see this:
And when I zoom in further, at the beginning of it:
So here is my question, if this is really because of arcing, I would like to understand this waveform better.
I imagine at the beginning the voltage increases because the contacts have already separated at this point and the inductive load is building up the voltage as the parasitic capacitance gets charged up by it. Then when the trace comes vertically straight up is where the arc occurs. The ringing that follows is either an artifact from the measurement or some kind of natural oscillation in the circuit like a spark-gap transmitter but I'm not too worried about this. I assume the arc extinguishes right when the voltage starts to build up again.
So the thing I don't understand what are the flat spots. Later on in the waveform they become really pronounced, take a look:
It appears to me that during the flat spots, the contacts have about -160V across and are just sitting there and then out of the blue it decides to arc again? Why wouldn't the voltage keep increasing and then once it reaches a maximum then right away the vertical line happens? Or maybe I'm completely misunderstanding what's going on and any help would be greatly appreciated!